A common method of drying lightweight paper web products after manufacture of such webs is to adhere the wet web to the exterior surface of a steam heated Yankee dryer. A Yankee dryer is a cylindrical dryer that is heated internally by way of steam that passes through its interior, thereby supplying heat energy to the exterior surface of the dryer. While adhered to the Yankee dryer, supplemental drying is provided by an air impingement hood that blows high velocity, heated air onto the exposed surface of the paper web. The air must be provided at relatively high velocity and extremely high temperatures to adequately dry the wet web in the short amount of time that the wet web is exposed to the air.
Using a standard wet pressed paper manufacturing operation, the level of adhesion to a Yankee dryer by the wet web is generally high enough to prevent disruption of the sheet during the drying process. In many other manufacturing processes, the wet web may be undesirably disrupted (i.e. damaged) by the force of high velocity air impinging on the wet web. Disruption of the wet web causes portions of the sheet to come loose from the surface of the Yankee dryer. This leads to scorching of the paper web from the high temperature impingement air and ultimately to plugging of the hood causing fires.
Impingement nozzles apply air at high speed from a drying hood to the surface of a cylindrical dryer. In general, the space between the wet web on the cylindrical dryer and the hot air hood is only a few inches.
What is generally needed in the industry is an apparatus, system, and method that is capable of adequately drying the wet web while it rotates on a cylindrical drum, below the dryer hood. A system of applying hot air at useful velocities to adequately dry the web, while also avoiding disrupting the web on the surface of the dryer, is needed. Furthermore, a system that can achieve these results while stabilizing the wet web on the surface of the drying cylinder would be particularly useful.
In the application of this invention, several different embodiments are possible, and this specification is intended to show multiple embodiments. Other embodiments not shown herein are possible as within the spirit of the invention.
A web drying method is presented, the method comprising providing a rotating steam-heated drying cylinder having an exterior surface. Further, a drying hood is located adjacent the exterior surface of the drying cylinder, the hood comprising a drying zone of heated air, the hood further comprising nozzles adapted to supply hot air streams into the drying zone. A step of applying a wet web to the exterior drying surface of a cylinder is provided. Furthermore, a web is rotated upon the exterior surface of the cylinder through the drying zone of the hood. Hot air streams are directed from the nozzles upon the surface of the wet web. At least one hot air stream is provided from a nozzle at a lead angle that is deviated from perpendicular between about 3 and about 30 degrees.
In a further method of the invention, the web is secured to the surface of the cylinder while substantially avoiding disruption of the web upon the surface of the cylinder. In some instances, the lead angle is deviated from perpendicular between about 5 and 20 degrees. The lead angle, in other embodiments, is deviated from perpendicular about 10 degrees.
The method also is provided in which the web is traveling at a speed, relative to the hood, of between about 1,000 and 6,000 feet per minute. In some instances, the speed of the air stream exiting the nozzle is between about 20,000 and 30,000 feet per minute.
For some applications of the invention, the temperature of the hot air stream is between about 500 degrees Fahrenheit and about 1100 degrees Fahrenheit.
The nozzle direction may be angled towards the edges of the web from the midline, at a compound angle. That is, in some applications of the invention, the angle of the nozzles is deviated in two directions, one direction leading the web (alpha angle), and one direction along the axis of the rotating dryer (beta angle).
A method also is provided in which a hot air stream is directed at a deviation angle beta from perpendicular that increases as the nozzle distance from the midline increases. That is, the angle is greater out near the edges of the web, and smaller near the midline. There would be no compound angle (i.e., beta angle) at all at the midline, in most cases.
A web drying apparatus is provided in another embodiment of the invention. The apparatus comprises a rotating drying cylinder, the cylinder having an exterior drying surface adapted to receive a wet web. Further, the apparatus includes an air hood, the air hood comprising an air blowing means capable of providing a stream of high speed heated air upon a wet web on the exterior surface of the drying cylinder. A plurality of air nozzles are connected to the air hood, the air nozzles being oriented to eject air from the hood towards the drying cylinder at a predetermined angle alpha, the angle alpha being deviated from perpendicular between about 3 and about 30 degrees. Depending upon the web rotational speed, and the air speed, the appropriate lead angle of the nozzles may be only about 5-15 degrees.
In some applications of the invention, there is a deviation of the nozzle angle along both the alpha direction and a beta direction. That is, in some applications of the invention, the web further comprises a midline along its middle with a left and right edge, further wherein nozzles of the hood located on either side of the web midline. The nozzles eject hot air upon the web, wherein the direction of air ejected is angled towards the edge of the web in both the left and right directions. The nozzles on the left of the web generally are angled to the left edge of the web, while the nozzles located to the right of the web midline are directed or angled to the right edge of the web. The angle in the beta direction is known as a compound angle.